The present invention relates to a method and apparatus for active power factor correction, and, more particularly, to an improved method and apparatus for critical mode and discontinuous mode control of boost converters.
Power factor is a measure of the efficiency of electrical utilization by electrical loads. Because instantaneous electrical power equals the current multiplied by the voltage, an electrical load which draws a large current at a low voltage and which sustains a high voltage while drawing little current does not utilize the power delivery capabilities of its power supply efficiently. For maximum efficiency, a load should always draw a current that is proportional to the voltage applied across the load. That is, the load should appear as a resistive impedance to the power supply. For a purely resistive load, the power factor equals unity, whereas for loads which depart from this ideal behavior the power factor will be less than one. A low power factor is undesirable from a standpoint of equipment and power main utilization, and in most cases it is necessary to provide means of increasing a low power factor toward unity.
For inductive or capacitive loads, power factor correction is usually accomplished by the use of passive components which eliminate high-order harmonics and reduce the first harmonic phase difference between the voltage and current to as close to zero as possible. At higher frequencies, however, the problem of total harmonic distortion becomes more important than phase shift, and active electronic means are needed to perform the correction. This is especially true in cases involving non-linear loads. Power supplies, for example, particularly high-frequency power supplies, create severe total harmonic distortion which leads to a low power factor.
This problem associated with total harmonic distortion and the reduction of power factor is illustrated in FIG. 1, to which reference is now briefly made. FIG. 1 illustrates the input voltage and current waveforms as functions of the time t for a device which rectifies alternating current and charges a capacitor. This is a common configuration for power supplies. In FIG. 1 a full-wave rectified input voltage V.sub.in which has been smoothed by a capacitor produces a filtered voltage waveform 2. When input voltage V.sub.in falls, the capacitor sustains the output voltage during a capacitor discharging interval 4. When input voltage V.sub.in rises, however, the capacitor charges during a capacitor charging interval 6. During capacitor charging interval 6, an input current I.sub.in exhibits highly-distorted sharp-peak current drains. The resulting waveform of input current I.sub.in therefore is not proportional to input voltage V.sub.in, and associated with the total harmonic distortion caused by the sharp pulses of input current I.sub.in is a reduced power factor. The goal of active power factor correction in such cases is to shape the input current waveform to have the same shape as the waveform of the input source voltage.
A common means of shaping the input current waveform to be proportional to the input voltage waveform is to use a boost converter. The basic topology of prior art boost converters is shown in FIG. 2, to which reference is now made. This topology is referred to as the "boost topology."
A boost converter is a type of DC-to-DC power converter whose output voltage at a load Z.sub.L is higher than the voltage of the power source V.sub.in connected to the input. It operates through cycles of charging and discharging energy into inductor 10 in the form of inductor current I.sub.in. A small input capacitor C.sub.in 13 is used to filter the input current, and a large output capacitor C.sub.out 15 is used to keep a constant output voltage. The operating cycle starts when a boost switch 14 is closed and current I.sub.in from the power source V.sub.in starts to build up the magnetic field in inductor 10. When the current reaches a value as determined by an external controller (not shown in FIG. 2), boost switch 14 is opened, at which point the collapsing magnetic field of inductor 10 generates a voltage at a point 11. When the current I.sub.in collapses to a value determined by the external controller, or if boost switch 14 was opened for a time determined by the external controller, boost switch 14 is closed and the cycle starts again.
The present application uses the terms "closed" and "on" equivalently to denote a state of a switching device wherein the switching device conducts electric current, and uses the terms "opened" and "off` equivalently to denote a state of a switching device wherein the switching device does not conduct electric current. The present application uses the term "closing" to denote the action of putting a switching device into the closed state, and uses the term "opening" to denote the action of putting a switching device into the opened state. The present application uses the term "inductive charging" to denote the driving of current through an inductor by an external voltage in order to build a magnetic field. The present application uses the term "inductive discharging" to denote the driving of current by an inductor due to the collapse of the inductor's magnetic field. Once boost switch 14 is opened, the current driven by inductor 10 flows into load Z.sub.L. Inductor 10 is able to generate voltages in excess of V.sub.in, which can result in the storage of energy in load Z.sub.L if load Z.sub.L has a reactive component. Therefore, a diode 12 is placed before an output capacitor C.sub.out 15 in order to prevent current from flowing out of load Z.sub.L and back into inductor 10. Although the relationship between output voltage V.sub.out and output current I.sub.out is determined by the nature of load Z.sub.L, the power factor is not concerned with the output parameters of the boost converter, but rather with the input parameters of the boost converter. Therefore, by carefully regulating the closing and opening of boost switch 14, it is possible to shape the waveform of input current I.sub.in so that input current I.sub.in is always proportional to the waveform of input voltage V.sub.in, and thereby to keep the power factor at or near unity.
It is noted that the boost topology relies on the principle of pulse-width modulation, wherein the duty cycle of the boost switch is altered by varying the respective time intervals of the pulses corresponding to the closed and opened states. The present application uses the term "duty cycle" to denote a repeated sequence of two alternating states, each state of which has a specified time interval. The present application uses the term "pulse-width modulator" ("PWM") to refer to any circuit which performs pulse-width modulation. The present application uses the term "time interval" to denote a period or duration of time as distinct from the occurrence of an instant of time. Both the start and end of a time interval are instants of time, and the time interval is the period of time between them.
There are three principal operating modes for a boost converter. operating modes are distinguished by how current flows through inductor 10 (FIG. 2). One operating mode is referred to as the "continuous mode," wherein input current I.sub.in flows without interruption in the same direction through inductor 10. The waveforms of the continuous mode are illustrated as functions of time t in FIG. 3 for an input current I.sub.in which closely follows the waveform of a rectified voltage V.sub.in. Although current I.sub.in flows continuously, the magnitude and slope of current I.sub.in with respect to t will change sign as boost switch 14 (FIG. 2) is opened and closed. A second operating mode is referred to as the "discontinuous mode," wherein input current I.sub.in periodically stops flowing and remains off for a certain time interval. The waveforms of the discontinuous mode are illustrated as functions of time t in FIG. 4 for an input current I.sub.in which closely follows the waveform of a rectified voltage V.sub.in. Between each activation of boost switch 14 (FIG. 2), there is a time interval during which input current I.sub.in =0. The third operating mode is referred to as the "critical mode," wherein input current I.sub.in falls to zero and then immediately begins to flow again. The waveforms of the critical mode are illustrated as functions of time t in FIG. 5 for an input current I.sub.in which closely follows the waveform of a rectified voltage V.sub.in. In the ideal case for the critical mode, even though input current I.sub.in decreases to zero at a point between each activation of boost switch 14 (FIG. 2), current I.sub.in immediately begins to increase so that there is no period during which input current I.sub.in remains zero. (The switching frequency is shown in FIG. 3, FIG. 4, and FIG. 5 as being of the order of the frequency of the input voltage V.sub.in for the purposes of illustration only, and in actual practice the switching frequency is much higher than depicted in the figures.) In all three of these operating modes the waveform of the average value of input current I.sub.in has the same shape as the waveform of input voltage V.sub.in and therefore has a power factor at or near unity. In the discontinuous mode (FIG. 4) and in the critical mode (FIG. 5), the instantaneous value of input current I.sub.in has a triangular waveform whose base is at zero current. The average value of input current I.sub.in at any point is given by the area of the triangular portions up to that point. For the critical mode, the triangular portions touch one another, and therefore the average input current I.sub.in for the critical mode is equal to half the height of the triangular portions. For the discontinuous mode, the triangular portions do not touch one another, and therefore the average input current I.sub.in for the discontinuous mode is less than half the height of the triangular portions.
The continuous mode is typically used for high power applications. A drawback of the continuous mode is that a high reverse current flows briefly through diode 12 and boost switch 14 (FIG. 2) until diode 12 recovers, thereby putting stress on diode 12. Another drawback of the continuous mode is that inductor 10 must have a high value of inductance. A further drawback of the continuous mode is that it generally requires an analog multiplier for regulating the cycle by which boost switch 14 is operated, and this adds complexity and cost to the implementation.
The discontinuous mode is useful at low power levels. Diode 12 recovers at zero current and there is therefore no stress on diode 12 and boost switch 14. However, the drawback to the discontinuous mode is that there is usually an associated high ripple current.
The critical mode is the most commonly-used operating mode for boost converters. As with the discontinuous mode, diode 12 (FIG. 2) recovers with zero current and therefore there is no stress on diode 12. The critical mode, however, has the capability of controlling higher power than the discontinuous mode and has a lower ripple current. The critical mode also has advantages over the continuous mode. The critical mode is easier to implement than the continuous mode, and the critical mode offers better feedback loop stability and can utilize a smaller inductor. In order to operate a boost converter in the critical mode, it is necessary to detect when input current I.sub.in reaches zero. The time when input current I.sub.in reaches zero is the instant at which boost switch 14 (FIG. 2) must be closed. The present application uses the term "forward inductor current" to denote the current which flows in an inductor in the direction preferred by the externally applied voltage, and uses the term "zero forward inductor current" to denote the condition which occurs when the instantaneous forward inductor current falls to zero. The present application uses the term "reverse inductor current" to denote the current which flows in an inductor opposite to the direction preferred by the externally applied voltage, and uses the term "zero reverse inductor current" to denote the condition which occurs when the instantaneous reverse inductor current falls to zero.
Unfortunately, although boost converters are able to perform power factor correction efficiently for a variety of loads, certain deficiencies of the prior art zero-detection method for critical mode operation lead to a residual total harmonic distortion at low power levels, caused by parasitic oscillations resulting from parasitic capacitance in the components of the boost converter. These low power levels occur at small loads, and they also occur during the normal operating cycle at the points when the input voltage V.sub.in waveform (FIG. 5) comes near zero volts. When total harmonic distortion increases from this effect, the power factor correction becomes less efficient. Therefore, it would be highly advantageous to have a method and apparatus which is able to eliminate the residual total harmonic distortion that occurs at low power levels by compensating for the parasitic capacitance in the components of the boost converter. This goal is met by the present invention.
Furthermore, the techniques used in the prior art to implement a zero current detector are not wholly satisfactory. For example, inductor 10 (FIG. 2) may be provided with a secondary coil, to function as a flyback transformer for sensing the collapse of the magnetic field and thereby providing a zero current detector which can detect zero forward inductor current (J. H. Alberkrack and S. M. Barrow, "An Economical Controller Corrects Power Factor", Power Conversion, September 1992 Proceedings, p. 322-329). Techniques such as this, however, can involve significant additional expense in a low-cost circuit for power factor correction, and moreover are limited to detecting the zero forward inductor current. To compensate for the effect of the parasitic capacitance of a boost converter, it is desirable to also detect zero reverse inductor current. Therefore, it would be highly advantageous to have a less expensive method for implementing a zero current detector which can detect both zero forward inductor current and zero reverse inductor current. This goal is also met by the present invention.
Moreover, it is desirable to be able to utilize certain types of digital circuits in pulse-width modulators because of their versatility and low cost. In particular it is highly desirable to be able to utilize clocked digital circuits, including but not limited to microprocessors and microcontrollers, in pulse-width modulators. The present application uses the term "clocked digital circuit" to denote any electrical circuit which is synchronized by, triggered by, or which otherwise operates in accordance with discretely timed clock signals that repeat at regular time intervals. Unfortunately, although clocked digital circuits are able to output control signals having durations of calculated time intervals, the calculated time intervals output by a clocked digital circuit are constrained to always be multiples of a basic time interval which is equal to the period of the clock which drives the clocked digital circuit multiplied by the number of clock cycles required for the relevant operations. The present application uses the term "resolution" to denote the minimum time interval which a clocked digital circuit can output. For example, if a clocked digital circuit has a clock period of 500 nanoseconds and requires 10 clock cycles to perform a particular timing operation, then the time intervals which can be output by this clocked digital circuit will be limited to time intervals which are multiples of 5 .mu.sec, and hence the resolution of this clocked digital circuit is 5 microseconds. Such a clocked digital circuit is able to output time intervals such as 5 microseconds, 10 microseconds, 15 microseconds, 20 microseconds, and so forth, but is not able to output time intervals such as 6 microseconds, 13.5 microseconds, 17 microseconds, or 19 microseconds. For effective power factor correction, however, it may be necessary to be able to adjust the time intervals of a duty cycle to a finer resolution than an otherwise suitable clocked digital circuit can attain. This goal is also met by the present invention.